The vast majority of additive manufacturing methods rely on what is often described as a “layer-by-layer” approach to producing parts of the object to be realized. In reality, these processes are actually not layer-by-layer, as each layer must be constructed from a series of linear or curvilinear paths, who are themselves constructed from a series of points. These methods therefore rely on an inherently hierarchically serial approach, where each successive portion of the object cannot be produced until previous portions are completed. Thus, utilizing these approaches amounts to constructing three-dimensional objects serially, a single point at a time.
Existing additive manufacturing processes typically require several components including:
(1) A virtual representation of some predetermined geometry to be fabricated;
(2) Some algorithmic means for discretizing that geometry into respective two-dimensional layers depending on local object cross-section with accompanying “tool paths,” i.e. a “slicer;” and
(3) An apparatus that deposits mass and/or energy locally that takes advantage of some material transformation, in a spatially resolved and controlled manner according to the calculated tool paths.
Integration of these three components enables the hierarchical point-by-point, path-by-path and layer-by-layer building of a generalized geometry used in current techniques.
FIGS. 1A-1C depict aspects of three illustrative additive manufacturing processes in accordance with the prior art. The dominant techniques currently in use include those that are based on deposition techniques such as Fused Deposition Modeling (FDM), aspects of which are illustrated in FIG. 1A; those that are based on sintering techniques such as Selective Laser Sintering (SLS), aspects of which are illustrated in FIG. 1B; and those that are based on photo-activation techniques such as stereolithography, aspects of which are illustrated in FIG. 1C. All of these techniques rely on the use of a point-by-point paradigm for realizing physical objects from virtual models, and are inherently serial in nature since successive paths and/or layers may only be produced after the preceding ones are completed.
For example, FDM, described in U.S. Pat. No. 5,121,329 and illustrated in FIG. 1A, typically relies on the use of polymer filaments which are extruded through a heated orifice to soften the material above its glass transition temperature, with deposition on a platform using a Cartesian mechatronic motion system to realize the path/layer geometry. Upon deposition of a material layer, the material cools, fully hardens, and adheres to the platform (in the case of the first layer) or the preceding layer (in the case of subsequent layers). However, due to the limitations of polymer materials, the mechanical performance for most FDM-produced components is not sufficient for application in mechanically demanding environments. In addition, the planar spatial resolution for this process is typically on the order of 200 um, and so this process is not suitable for making fine-scale features. Although there have been some efforts at using metal wire filaments to generate metal objects, the temperature requirements for melting metals is significantly higher than for polymers, and the atmospheric control needed to avoid deleterious oxidation have resulted in a reduced use for most commercial and consumer applications.
SLS processes, described in U.S. Pat. No. 4,863,538 and illustrated in FIG. 1B, are based upon using lasers to locally melt or sinter polymer or metal powder precursors in order to create cross-sections for layer-wise three-dimensional object creation. The ability of this process to create metallic components has facilitated its adoption across a much wider range of industries and applications. It exhibits the fine spatial resolution enabled by its use of a laser source, while also maintaining its ability to produce components with far greater mechanical performance than FDM or stereolithography (described below), due to its ability to process a wider range of material systems. However, as the melting or sintering process occurs relatively rapidly, the resulting components created tend to exhibit many types of micro, meso, and macro-scale flaws, including significant degrees of porosity, microstructural defects, residual stress, cracks, and warpage. These flaws result in components whose performance is significantly degraded relative to their fully dense/traditionally fabricated counterparts. Post-processing steps are also frequently required before objects can be employed in a functional manner. A related technique, known as Electron Beam Melting (EBM) functions in a manner virtually identical to SLS, except for the use of an electron-beam energy source (in vacuum) instead of a laser. See Alderson Neira Arce. Thermal Modeling and Simulation of Electron Beam Melting for Rapid Prototyping of Ti6Al4V Alloys. Ph.D., North Carolina State University, 2012.
Stereolithography, described in U.S. Pat. No. 4,929,402 and illustrated in FIG. 1C, uses lasers to photopolymerize and harden photocurable liquid resins in order to build three-dimensional objects, again through a layer-by-layer approach. A major advantage of stereolithography is that its resolution is far greater than FDM, as it is limited primarily by the laser spot size and thermo-viscosity of the liquid, allowing the creation of much finer features with greater fidelity. The drawback of this method is that the material systems in which it can be used are extremely limited and do not exhibit sufficient mechanical performance for most structural applications, though there have been recent efforts using pre-ceramic-based polymers that are photo-activated, resulting in ceramic parts that have superior properties to those manufactured using common photopolymers. See Z. C. Eckel, C. Zhou, J. H. Martin, A. J. Jacobsen, W. B. Carter, and T. A. Schaedler, “Additive manufacturing of polymer-derived ceramics,” Science, 351(6268):58-62, 2016.
It is also noted that while the common techniques described above adhere to an ultra-serialized approach, a point by point building of an object, there have been efforts at generating objects in a truly layer-by-layer fashion. The so-called “Continuous Liquid Interphase Printing” (CLIP) process, described in U.S. Pat. No. 9,360,757, employs the time-varying projection of a two-dimensional image on a continuous, vertically translating build platform to photo-polymerize cross-sectional layers for subsequent object creation. Another technique for creating objects through true section-wise construction is the so-called “laser decal transfer” process described in U.S. Pat. No. 8,728,589, which utilizes high viscosity “nano-inks” that can preserve the geometry of the laser beam used to propel a portion of material on to a substrate for building objects, typically at micron to millimeter scales. Finally, recent work detailed in Shusteff et al. (“Additive Fabrication of 3D Structures by Holographic Lithography,” Proc. of 27th Int. Solid Freeform Fabrication Symp., 2016) demonstrated the ability to use holographic lithography to create entire objects simultaneously. However, the maximum object sizes are limited to less than 1 cm, and suffer from the same material limitations as those described for use in stereolithography.
The resulting multi-scale stratification of mass and accompanying complex thermal histories introduced by such hierarchical processes have significant problems with respect to scaling and build times, as well as introducing weaknesses such as structural anisotropy, microstructural defects, mesoscopic deficiencies, and macroscopic geometric deviations in the resulting objects.